When you get a question with "at least" in, it is often a good idea to find the probability of "none" first, then the probability of "at least" = 1 - probability of "none".
Suppose there are n integers between 1 and 50 that are not multiples of 3. Then the probability of choosing a random integer that is not a multiple of 3 is n/50.
The probability of doinjg this twice is (n/50)^2. Hence the probability that at least one number is a multiple of 3 is 1 - (n/50)^2